Surface discharge multiple gap sparking plug



Oct. 10, 1961 J. N. M. DEANS 3,004,184

SURFACE DISCHARGE. MULTIPLE GAP SPARKING PLUG Filed D00. 5, 1958 INVENTOR. James N. M. Deans ATTORNEY United States Patent 3,004,184 SURFACE DISCHARGE MULTIPLE GAP SPARKING PLUG James N. M. Deans, Renfrew, Scotland, assignor to Babcock & Wilcox Limited, London, England, a company of Great Britain Filed Dec. 5, 19 58, Ser. No. 778,357 3 Claims. (Cl. 313 123) This invention relates to igniting means of the surface discharge sparking plug type for furnaces fired with fluent fuel, particularly liquid fuel, and more particularly to improved surface discharge sparking plug type igniters and methods of manufacturing the same.

In accordance with the present invention, a surface discharge sparking plug is provided comprising a series of spaced electrodes, with dielectric or semiconducting ma terial providing respective surfaces extending across the gaps between adjacent electrodes of the series and providingserially arranged surface discharge paths between the electrodes of the series.

More particularly, the sparking plug comprises a central conductor in the form of a metal rod extending coaxially from a hollow cylindrical end housing in which it is supported by a dielectric cap or plug. The free or forward end of this conductor has electricflly and mechanically connected thereto an end electrode which is substantially cup-shape, including a circular disk portion having a rearwardly directed cylindrical flange. A plurality or series of annular electrodes are arranged coaxially of the conductor rod and in axially spaced relation to each other and to the flange of the end electrode, the rearmost annular electrode seating in and being electrically and mechanically secured to the end housing.

In one embodiment of the invention, a tube of dielectric or semi-conducting material is telescoped through the annular electrodes and into the flange of the end electrode, and its outer surface provides the surface discharge paths between the series of electrodes.

In another embodiment of the invention, the inner peripheral surfaces of the flange of the end electrode and of the annular electrodes are formed to seat axially spaced annular disks or spaces of dielectric or semi-conducting material spacing the electrodes axially from each other and having outer surfaces providing the surface discharge paths between the series of electrodes.

In both embodiments, the facing edges of adjacent electrodes, outwardly of the outer surfaces of the tube or disks of dielectric or semi-conducting material, may be parallel and lie in radial (diametric) planes, or may converge toward each other in a radially outward direction.

A preferred means for energizing the igniter comprises a capacitor charge-discharge circuit including a cyclically operable make and break switch.

The first-mentioned embodiment of the sparking plug is manufactured by shrinking a hollow metal cylinder or tube of electrode material onto an inner tube of dielectric or semi-conducting material and then removing metal at axially spaced locations to expose the surface of the inner tube and form the axially separated annular electrodes.

The second-mentioned embodiment of the sparking plug is assembled by placing the spacer disks and annular electrodes on the rod conductor and then threading a nut on the rear or outer end of the latter to draw the disks and electrodes into a tight unit. The inter-electrode spacing may be set by selecting spacer disks of the requisite thickness. This ready interchangeability feature is an outstanding characteristic of this embodiment of the invention For an understanding of the invention principles, reference is made to the following description oftypical emice bodiments thereof as illustrated in the accompanying drawings. In the drawings:

FIG. 1 is a sectional side view of one embodiment of the invention in which the sparking plug is provided with a coaxial series of spaced annular electrodes peripherally embracing a cylinder of insulating material;

FIG. 2' is a sectional side view of an alternative embodiment in which the sparking plug has a plurality of elec-. trodes respectively separated by individual spacers of insulating material;

FIG. 3 is a detail view to an enlarged scale showing a shape of electrode alternative to that shown in FIGS. 1 and 2; and 7 FIG. 4 is a schematic wiring diagram of a preferred energizing circuit for the sparking plugs.

Referring to FIG. 1, the sparking plug therein illustrated comprises a central rod or conductor 1, extending coaxially within a hollow cylindrical metal end housing 8 and a cylindrical tubular part 3 of dielectric or semi-conducting material. At its inner or forward end remote from the housing 8, the rod 1 is electrically and mechanically connected, suitably by welding, to an end electrode 2 formed as a disk with an annular peripheral flange 2 tightly engaging the outer periphery of the end of the tube 3.

Annular electrodes 4, 5 and 6 are provided at spaced intervals along the tube 3, all tightly engaging the outer periphery of the tube, and the last electrode 7 is longitudinally extended to cooperate with the bore of the housing 8. Thus the last or outermost electrode'is fitted within such bore and is maintained in position and in good electrical and mechanical connection with the housing ,8 by means of an annular deposit 9 of weld metal.

The sparking plug is suitably formed by shrinking a cylindrical metal tube or sleeve of electrode material onto a cylindrical tube of dielectric or semi-conducting mate rial and machining away electrode material at longitudinally spaced intervals to provide serially arranged annular gaps 15, 16, I7 and 18 circumscribing the tube 3 and revealing the outer peripheral surface thereof, and to form the discharge surfaces 15, 16, 17' and 18' and the spaced series of electrodes 2, 4, 5, 6 and 7. As a dielectric material for the tube there may be used unglazed porcelain, or as semi-conducting material, carbon impregnated porcelain or a suitable cermet may be used.

The end electrode 2 is suitably formed as a disk of electrode material with a central bore into which is fitted anend of the rod 1, and the radially outer periphery of the disk is welded to the flange 2' and the radially inner eriphery'of the disk is Welded to the end of the rod 1, I

so closing the end of the tube 3.

The housing 8 is relatively short and is suitably adapted to connect with a tubular part for connection to the energizing circuit, the tubular part being of such length as: to support the sparking plug in desired position with reference to the spray of fuel from a fuel burner. The housing may, however, be of any desired length, and may be sufficiently long to support the sparking plug in desired position with reference to the spray of fuel from an oil burner.

The outer end of the housing 8 is provided with an end cap or plug 1 1 of insulating material having an inner portion adapted to fit Within the bore of the housing 8 and an outer flange portion adapted to seat against the outer end face of the housing 8.

The cap or plug is provided with a central bore through which passes the outer end 12 of the conductor rod 1. The outer end 12 of the conductor rod 1 is threaded to accept a cap retaining washer 13 and nut 14 and provides a terminal for connection to the energizing circuit.

In operation, the sparking plug may be subject to intense heating and to this end the electrodes 2, 4, 5, 6 and 7 are formed from heat resistant metal, suitably a stainless steel, and inner tube 3 of a ceramic, suitably unglazed vitreous refractory material.

In the operation of a surface discharge sparking plug the arc formed between adjacent electrodes initially tracks across the discharge surface therebetween. The discharge surface is covered with carbonaceous material e.g. from fuel; serving to render possible the operation of the sparking plug when an insulating material is used in its construction. The formation of the are at the discharge surface produces different results, one of which is that liquid fuel trapped between the electrodes is cracked by the heat of discharge, complete combustion not being possible due to lack of air in the gap, and carbonaceous material is deposited on the relatively cool surface of the insulaitng material so that material eroded by the discharge is constantly replaced.

A further effect, which may be due to the formation of gas or vapor at the bottom of the gap, is that the are moves outwardly between the electrodes and away from the discharge surface so that erosion thereof is minimized. A still further effect, depending on the immersion of the surface in the liquid or oil fuel, is that the fuel in the gap is violently and finely atomized and ejected from the gap to create a first or pilot flame which effects ignition of the main liquid fuel spray from the burner.

The tube 3 of dielectric or semi-conducting material may readily be finished with the minimum of surface flaws and presents favorable surface discharge paths in that surface erosion due to repeated tracking of a spark across a flaw is minimized, and in that the spark may more readily take place at various positions around the tube peripheral surface.

In the sparking plug illustrated in FIG. 2, a central rod or conductor 21 is welded at its inner end to an electrode 22. The electrode comprises a disk provided with an outer annular flanged part 22 having a radially inner shoulder 22" adapted to cooperate with the outer peripheral edge of a disk-shaped spacer 23 of dielectric or semiconducting material. The electrode 22 is bored to fit onto the end of the rod 21 and is retained in position and in good electrical contact with respect to the red by an annular deposit 3-1 of weld metal.

Slidably mounted on the rod 21 are four annular spacers or disks 23, 25, 27 and 29 of dielectric or semiconducting material, respectively acting as spacers between pairs of adjacent electrodes 22 and 24, 24 and 26, 26 and 28, 2S and 30 to form the serially arranged spark gaps 39, 40, 41 and 42. The electrodes 24, 26 and 28 are formed as annular metal parts of electrode material, and at opposite sides the annular electrodes are formed with inner peripheral rabbets adapted to cooperate with outer peripheral parts and respective side faces of the associated spacers. Thus, the annular electrode 26 is formed at opposite sides with inner peripheral rabbets 26 and 26" respectively adapted to cooperate with the outer periphery 25 and the side face 25" of the spacer 2S and with the outer periphery 27 and the side face 2 of the spacer 27. In this way the annular electrodes are radially and longitudinally located by the spacers.

At the outer end of the sparking plug, the electrode 30 is formed as a tubular part provided at its inner end with an inner peripheral rabbet to cooperate with the outer peripheral surface 29 and the outer side face 29" of the outermost spacer 29, and adapted at its outer end to fit within the bore of a hollow cylindrical housing '33. The housing 33 is suitably electrically and mechanically connected to the electrode 30 by an annular weld deposit 32, and its outer end 34 is threaded to cooperate with a connecting part for connecting the outer electrode 30 to an energizing circuit. Alternatively the end electrode 30 may be formed integrally with the housing 33.

A cap or plug 35 is provided at the outer end of the housing 33 adapted at one part to fit within the bore of the housing and at another part to abut the outer end face of the housing so as to locate the plug in position. The rod or conductor 21 extends axially through the housing 33, passing through a central bore in the cap or plug 35. The plug is retained in position by a retaining washer 36 and nut 37 engaging the threaded end portion 33 of the rod 21, and the end portion 33 of the rod provides a terminal for connection to the energizing source.

The width of the spark gaps 3), 4t), 41 and 42 between adjacent electrodes may be adjusted by interchanging the associated spacers 23, 25, 27 and 29 with similar spacers of different thicknesses. Thus the retaining nut37 may be removed, and the rod 21 withdrawn from the housing 33, the electrodes 24, 26, 28 and the spacers 23, 25, 27 and 29 withdrawn from the rod, and fresh spacers of the desired thickness slid onto the rod between the asso ciated electrodes. The plug may then here-assembled by inserting the rod 21 through the housing 33 until the outer spacer engages the end electrode 30, inserting the plug 35 and, after replacing the retaining nut and washer, tightening the nut 37 until the associated spacers and electrodes abut.

Thus adjustment may conveniently be made to obtain the optimum discharge path length for different fuels. Similarly worn or eroded electrodes may readily be replaced.

In forming the sparking plugs shown in FIGS. 1 and 2, opposing faces of adjacent electrodes are made parallel to one another so that suitably the faces extend in planes normal to the longitudinal axis of the plug and to the associated discharge surfaces. With such arrangement, carbonaceous material is more readily deposited on the surface of the tube 3 of spacers 23, 25, 27, 29 than is the case with opposed electrode faces which diverge radially outwardly.

Thus in FIG. 1 opposed faces of the electrode pairs 2 and 4, 4- and 5, 5 and 6, and 6 and 7 are parallel and similarly in FIG. 2 opposed faces of the electrode pairs 22 and 2-4, 24 and 26, 26 and 28, and 28 and 30 are parallel.

Alternatively, as shown in FIG. 3, the opposing faces of adjacent electrodes may be made convergent in a direction away from the associated discharge surface to minimize the danger or". removal of the carbonaceous material by the explosive nature of the discharge.

Thus electrodes 61 and 62 mounted on the tube 63 are spaced apart by the surface 64 of the tube and are formed with respective opposed faces 65 and 66 radially outwardly convergent so that the gap between the electrodes 61 and 62 is wider at the surface 64 than at the radially outer periphery 67 of the plug.

Suitably spark plugs of the nature described with reference to FIGS. 1, 2 and 3 form part of igniting means, for effecting lighting of a liquid fuel burner, provided with means as are disclosed in British patent specification 780,653, corresponding to US. application Serial No. 507,287, filed May 10, 1955, which is now U.S. Patent 2,921,239 granted January 12, 1960, for positioning the plug in relation to the burner with a discharge surface or surfaces located to receive spray particles of liquid fuel from the burner, and means, suitably an energizing circuit as shown in FIG. 4, for effecting electrical discharges between the electrodes.

In the energizing circuit, an alternating current source is connected at terminals 53 to one side of a transformer 54, the other side of the transformer being connected to a charging circuit. The charging circuit includes a voltage doubler unit and rectifying circuit made up of two arms, one including rectifier 44 and condenser 46 in series and the other including rectifier 43 and condenser 45 in series. The charging circuit also includes a resistor 47 and capacitor 48 in series connection across the capacitors 45 and 46 of the doubler unit, and the capacitor 48 is connected to ground 49 on its side remote from the resistor 47.

The output of the voltage doubler is thus applied to the series combination of the resistor 47 and capacitor 48 to effect charging up of the capacitor 48.

The sparking plug, for example, of the nature shown in FIG. 1, comprisin the multiple gaps 15, 16, 17 and 18, the central conductor rod 1 and the housing 10, is connected by terminals 52 across the capacitor 48 in series with a switching device comprising the contact elements 50 and the make and break switching mechanism 51.

In operation, the switching device controls the discharge of the capacitor 48 across the multiple gaps 55 of the sparking plug by periodically closing the contact elements 50.

The charging circuit disclosed is suitably energized from a relatively low voltage source such as 240 volts 50 cycle per second alternating current.

A sparking plug according to the invention is more easily disposed in position for igniting the spray of fuel from an oil burner. The spark is distributed over a greater overall length than that of a single gap plug, so that the position of the sparking plug relative to the spray from a burner is less critical.

Furthermore, the total electrical energy dissipated across the series of gaps is the same as for a similar single gap plug, and consequently th metallic errosion at each of the gaps is reduced compared with that of a single gap plug.

- While specific embodiments of the invention have been shown and described in detail to illustrate the application of the invention principles, it will be understood that the invention may be embodied otherwise without departing from such principles.

What is claimed is:

1. A low voltage electrical igniting means for fluent fuel fired furnaces comprising a hollow cylindrical metal end housing, a hollow cylindrical dielectric tubular member co-axially connected to one end of said housing to form an extension thereof, said dielectric tubular member having a smooth and uninterrupted outer surface, a conductor rod extending co-axially along said housing and connected dielectric member in spaced relationship thereto, a dielectric bearing supporting one end of the rod on said housing, an electrode having an annular peripheral flange mechanically and electrically connected to the other end of said rod, said flange tightly engaging the outer periphery of said dielectric tubular member, a plurality of annular electrodes spatially disposed longitudinally of said tubular dielectric member, said annular electrodes defining therebetween a plurality of serially arranged annular sparking gaps, each of said gaps being formed between the facing lateral substantially parallel surfaces of adjacent electrodes extending laterally inward from the outer circumferential surfaces of the electrodes, and said annular electrode farthest removed from said flanged electrode being mechanically and electrically connected to said housing.

2. A low voltage electrical igniting means for fluent fuel fired furnaces comprising a hollow cylindrical metal end housing, a hollow cylindrical dielectric tubular member co-axially connected on one end of said housing to form an extension thereof, said dielectric tubular member having a smooth and uninterrupted outer surface, a conductor rod extending co-axially along said housing and connected dielectric member in spaced relationship thereto, a dielectric bearing supporting one end of the rod to said housing, an electrode having an annular peripheral flange mechanically and electrically connected to the other end of said rod, said flange tightly engaging the outer periphery of said dielectric tubular member, and a plurality of annular electrodes spatially disposed longitudinally of said tubular dielectric member, said electrodes defining therebetween a plurality of serially arranged annular sparking gaps, each of said gaps being formed between the facing lateral surfaces of adjacent electrodes extending laterally inward from the outer circumferential surfaces of the electrodes to said dielectric tubular member, and said facing surfaces of adjacent annular electrodes being convergent in a direction away from the outer surface of said dielectric tubular member.

3. A low voltage electrical igniting means for fluent fuel fired furnaces comprising a hollow cylindrical metal end housing, a conductor rod extending co-axially along said housing in spaced relationship thereto, said rod extending beyond said housing, a dielectric bearing supporting one end of the rod on said housing, a plurality of interchangeable annular dielectric members having an exposed smooth outer cylindrical surface spaced along the extended portion of said rod, an electrode having an annular peripheral flange mechanically and electrically connected to the other end of said rod, said flange tightly engaging the outermost annular dielectric member, and a plurality of annular electrodes spatially disposed longitudinally of said tubular. dielectric member, so that an annular dielectric member is disposed between and peripherally embraced by each pair of adjacent electrodes, said electrodes and annular dielectric members being loosely arranged about said rod, said electrodes defining therebetween a plurality of serially arranged annular sparking gaps, each of said gaps being formed between the facing lateral surfaces of adjacent electrodes extending laterally inward from the outer circumferential surfaces of the electrodes, and an adjusting means connected to said one end of said rod for drawing said electrodes and annular dielectric members into firm contact with one another.

References Cited in the file of this patent UNITED STATES PATENTS 1,270,437 Reppin June 25, 1918 1,359,358 Glassman et al. Nov. 16, 1920 1,471,309 Ehmke Oct. 16, 1923 2,260,399 Peters et a1. Oct. 28, 1941 2,689,556 Smits Sept. 21, 1954 2,745,980 Smits May 15, 1956 2,791,023 Stuermer May 7, 1957 2,864,152 Peras Dec. 16, 1958 FOREIGN PATENTS 1,141,080 France Aug. 26, 1957 

